A continuing concern in the manufacture of galvanic cells is that electrolyte may creep through a sealed interface of the cell and leak out of the cell. Electrolyte leakage may shorten cell life and can also cause a corrosive deposit to form on the exterior surface of the cell which detracts from the cell's appearance and marketability. These corrosive salts may also damage the device in which the cell is housed. Electrolyte leakage occurs in cell systems having aqueous or nonaqueous electrolytes, such as organic solvent-based electrolytes and liquid inorganic cathode-electrolytes, for example those based on thionyl chloride and sulfuryl chloride. Electrolytes such as alkaline electrolytes have an affinity for wetting metal surfaces and are known to creep through a sealed interface of a galvanic cell.
In the prior art it has been a conventional practice to incorporate an insulating gasket between the cell container and cover so as to provide a seal for the cell. Generally, the gasket must be made of a material inert to the electrolyte contained in the cell and the cell environment. In addition, the cell gasket must possess sufficient flexibility and resistance to cold flow under pressure as well as being able to maintain these characteristics so as to insure a proper seal during long periods of storage. Materials such as nylon, polypropylene, and high density polyethylene have been found to be suitable as gasket materials for most applications.
However, the use of a compressible gasket alone has not proved sufficient to reduce cell leakage to commercially acceptable standards. Accordingly, several approaches have been taken in the prior art in order improve the leakproofness of galvanic cells. Among the approaches which have been taken in the past is the deposition of a polymeric layer at the cell container/gasket and/or cover/gasket interface.
For example, U.S. Pat. No. 4,282,293 discloses an improved seal for alkaline cells wherein a film of a substituted organosilane is disposed and compressed between the interface of the cell cover and a coated gasket, such film being deposited utilizing a solvent which is subsequently evaporated. Preferably such film has a thickness of between 10 and 100 angstroms.
Japanese Patent Application No. 90146/1978 discloses the formation of a fluoropolymer film by sputtering or plasma deposition onto the surfaces of electrochemical cells in order to reduce electrolyte leakage. This patent application indicates that the thickness of the film deposited should be at least 3000 angstroms in order to avoid pinholes.
However, even these cell constructions are not successful in stopping cell leakage. Although not wishing to be held to any theory, applicants surmise that a major reason for the failure of the prior art ultra thin films to successfully prevent electrolyte leakage is that such films do not sufficiently adhere to the cell container and/or cover and/or gasket substrate.
It is believed that the resistance to chemical reactions which take place at the interface of a substrate and a polymeric coating and, hence the resistance of such interface to electrolyte leakage, can be considered to be dependent on four major factors: (1) the permeability of the coating to the electrolyte solvent; (2) the electrolyte repellency of the coating; (3) the adhesion of the coating to the substrate; and (4) the passivation effect on the interface caused by the coating. In the case of thick film coatings the permeability of the coating and the electrolyte repellence of the coating itself play predominant roles, with the third and fourth factors mentioned above playing a relatively minor roles.
However, in ultrathin film coatings, the role of the permeability factor becomes minimal due to the fact that the overall transport resistance is the ratio of the film thickness to permeability. For instance, water molecules pass through most organic polymers rather easily, and the time lag of diffusion of water through a 0.1 mm thick layer is well below a fraction of a second and it is nearly impossible to stop the penetration of water to the substrate/film interface. Consequently the adhesion characteristics, particularly the wet adhesion properties, become a vitally important factor in the overall chemical resistance of the interface. The adhesion and passivation effects of an ultrathin film are often closely related and are often inseparable characteristics of such a coating.
Thin films produced by evaporation methods, such as that employed in U.S. Pat. No. 4,282,293 are subject to pinhole formation, whereas films formed by sputtering or plasma polymerization do not generally contain pinholes. See Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 10 at page 275, John Wiley & Sons (3d Ed. 1980). Thus evaporation produced films such as those described in U.S. Pat. No. 4,283,293 will provide poorer protection for the substrate surface due to the presence of such pinholes. Moreover, such lower energy film deposition processes result in the polymeric film having a relatively poor adhesion to the substrate.
While Japanese Patent Application No. 90146/1978 employs higher energy means of coating the fluoropolymers on the cell surfaces, thin layers of about 300 angstroms of such fluoropolymers possess relatively poor maintenance of wet adhesion notwithstanding said application means as evidenced by the relatively poor wet adhesion exhibited by plasma polymerized tetrafluoroethylene and hexafluoroethane in Example 1 below.
A further problem which may be encountered during the lifetime of a galvanic cell is that of internal or external corrosion. Internal corrosion of interior corrodible cell members may undesirably increase the internal resistance of the cell, particularly during high temperature storage. External corrosion may detract from the cell's marketability or interfere wih the cell's contact with the device into which such cell is to be inserted. As is apparent to those skilled in the art, conventional coating of the external surfaces of the components of the cells housing or the internal surfaces of interior corrodible cell components with a protective layer is precluded as such layer will interfere with the cell's operation due to the high electrical resistance of such coating. Therefore, there is a need for a galvanic cell having internal and/or external surfaces which exhibit increased resistance to corrosion, which corrosion resistance means does not interfere with the effective operation of the cell.
Accordingly, it is an object of the present invention to provide a galvanic cell which exhibits improved resistance to electrolyte leakage.
It is another object of this invention to provide a galvanic cell which exhibits increased resistance to internal and/or external corrosion, which cell does not exhibit increased internal resistance or impaired contacts due to such corrosion resistance means.
It is a yet another object of this invention to provide a process for treating at least a portion of at least one surface of at least one component of a galvanic cell such that, when assembled, said cell will exhibit increased resistance to electrolyte leakage and/or to corrosion.
The foregoing and additional objects of this invention will become apparent from the following description and accompanying drawings and examples.